H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

So, I bought my Zenith 601 used. It has a Rotax 912 ULS 100HP. Since I've had it my tach hasn't shown more than about 5400 RPM at full throttle. Since we have changed out all of the fuel lines, re-installed and synced the rebuilt carbs, it's 5000 at full throttle. Now my understanding is it's supposed to make 5800 RPM at full throttle. My question is, is it possible that my tach is not accurate? Are there other reasons why the engine might not make 5800 RPM? Is it a question of prop pitch?

When throttle is full open, are carb throttle levers at their full open stop?When carbs were rebuilt, were all components (needle jet, jet needle, etc.) replaced with those exactly specified in the Rotax Parts Catalog for a 912ULS?

I am not familiar with the Zenith set up, but for a Flight Design I normally expect 4950 to 5000 static. Full throttle level flight somewhere between 5500 and 5650. For the CT this gives a pretty good all around balance between speed, climb performance, and fuel economy. Prop pitch is what we adjust to achieve that RPM. It sounds like you could use a prop pitch adjustment.

I would be more concerned about the fact that your RPM changed after maintenance. Beyond setting up something wrong or rigging of the carbs, to much oil on a K&N filter will starve the engine for air reducing RPM.

You can check RPM with a model airplane tachometer. Take the reading of the prop RPM and multiply by 2.43.

JJay wrote:When throttle is full open, are carb throttle levers at their full open stop?When carbs were rebuilt, were all components (needle jet, jet needle, etc.) replaced with those exactly specified in the Rotax Parts Catalog for a 912ULS?

The carbs were rebuilt at Lockwood(the previous owner had they done there the last time as well) and they also changed out the floats. Since we re-installed them we made sure they are hitting the stops at full throttle.

3Dreaming wrote:......I would be more concerned about the fact that your RPM changed after maintenance. Beyond setting up something wrong or rigging of the carbs, to much oil on a K&N filter will starve the engine for air reducing RPM.

You can check RPM with a model airplane tachometer. Take the reading of the prop RPM and multiply by 2.43.

My mechanic said he has something to double check the RPM. I have a few things to do yet, including cleaning the air filter. I'll make sure I'm a little stingy re-oiling it.

rcpilot wrote:Pardon my ignorance but if at WOT you are below the RPM cap, can you continue WOT as long as you want? Assuming my tach is accurate and I'm not reaching the cap.

Yes, but generally your ideal prop pitch setting will have you reaching 5600-5700rpm in level flight and WOT at your normal cruise altitude. This means you'll be running less than WOT in cruise between 5200-5500rpm. This will give the best climb/cruise compromise. If you want more climb go flatter/higher rpm, if you want a little more speed go closer to 5600rpm. But anywhere in that range should be good to go.

I favor higher rpm, and have had my prop as flat as 5750rpm WOT at cruise altitude. I'd rather have climb than cruise -- nobody ever got killed in an airplane because they had too much climb performance, though the converse is not true...